1. Filed of the Invention
The present invention relates to a method of elongating a glass preform for optical fiber.
2. Related Background Art
When making an optical fiber, in a step of disposing a cladding layer outside a core or a step of drawing the optical fiber as a final product, it is necessary to elongate a glass preform, which is an intermediate product of the optical fiber, to a predetermined outside diameter. As a method of precisely elongating the :glass preform for such a purpose, there have conventionally been known methods by use of a burner elongating machine disclosed in Japanese Patent Application Laid-Open No. 61-295,251 or No. 61-295,252. In these method, both end s of the glass preform are held with chucks, and these chucks are moved with a different speed while a part of the glass preform is heated and softened by the burner, so as to elongate the glass preform by a tensile force applied thereto. In these methods, the outside diameter in the tapered region of the glass preform in the process of elongating is measured, and the moving speeds of chucks are changed according to the difference between thus actually measured outside diameter and a preset outside diameter, i.e. outside diameter deviation, so as to carry out precision elongating to yield an elongated body with a predetermined outside diameter.
In the elongating methods disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 61-295,251 or No. 61-295,252, however, since the moving speeds of the chucks (chuck speeds) are changed according to the outside diameter deviation, the chuck speeds would be changed by a certain specific ratio, regardless of the magnitude of the preset outside diameter, when a certain particular outside diameter deviation occurs. For example, even in the cases with different preset outside diameters of 50 mm and 100 mm, their amounts of change in chuck speeds would be identical to each other when the same outside diameter deviation of 1 mm occurs. The deviation of 1 mm at the preset outside diameter of 50 mm is 2% in terms of ratio, whereas the deviation of 1 mm at the preset outside diameter of 100 mm is 1% in terms of ratio, whereby their influences upon the elongated body would differ from each other by a factor of 2, whereas the amount of control of chuck speed for correcting the deviation would be kept constant. Consequently, there has been a problem that, when preset outside diameters are different from each other, a difference may occur between effects obtained after the chuck speed control. Though this problem can be coped with by reviewing the control system every time when the preset outside diameter is changed, it requires an operation of modifying the control system on each occasion, thus being troublesome.
Also, in the elongating methods disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 61-295,251 or No. 61-295,252, the glass preform is heated and softened by a burner. Since the heating with a burner can be effected locally, it can narrow the melted area, thus being suitable for precision elongating. However, it cannot yield a sufficient amount of heat for softening a glass preform having a large outside diameter. Therefore, the glass preform having a large outside diameter has been elongated by an electric furnace elongating machine using such as a resistance furnace or induction furnace which can yield a sufficient amount of heat, to such an outside diameter that it can be precision-elongated by a burner elongating machine, and then has further been precision-elongated by the burner elongating machine.
Such a method, however, has been problematic in that a glass preform having such a large outside diameter that it cannot be elongated by a burner elongating machine cannot directly be precision-elongated, and in that two steps are employed, thereby necessitating time and cost upon making. In the extension by use of an electric furnace elongating machine, the heated and softened part becomes wider as compared with that obtained by a burner elongating machine, thus making it difficult to carry out the extension so as to yield a predetermined outside diameter. Therefore, the precision elongating has not yet been carried out by use of an electric furnace elongating machine alone.
As a result of diligent studies, the inventors have found that, by controlling extension process according to a specific value obtained from a reference outside diameter in a tapered region elongated by heating and softening and an actually measured one, precision elongating can be carried out by an electric furnace elongating machine alone even when the outside diameter of the glass preform before extension and the target elongated outside diameter vary, whereby the above-mentioned problems can be overcome. Based on this finding, it is an object of the present invention to provide a method of elongating a glass preform which can precision-expand the glass preform with a favorable accuracy and a favorable manufacturing efficiency regardless of the glass preform outside diameter and the target elongated outside diameter.
The method of elongating a glass preform in accordance with the present invention comprises the steps of holding both ends of the glass preform with first and second holding sections, respectively; with the moving speed of the first holding section faster than that of the second holding section; heating and softening the glass preform by a heating section successively from the end portion on the first holding section side as the first and second holding sections move in the longitudinal direction of the glass preform; and elongating the glass preform by means of a tensile force applied to the glass preform. In this method, an electric furnace is employed in the heating section. And the method further comprises the steps of setting a reference value R1 with respect to an outside diameter at a specific position in a tapered region of the glass preform in the process of elongating, acquiring an actually measured value R2 which is an outside diameter at the specific position and controlling the moving speeds of the first holding section and/or second holding section according to a value (R2/R1) obtained from the reference value R1 and actually measured value R2.
In the present invention, the moving speed of the first holding section and/or second holding section is not controlled by the difference between the reference value R1 and actually measured value R2 in the tapered region, i.e., the so-called outside diameter deviation, but by a value (R2/R1) obtained from the reference value R1 and actually measured value R2. Consequently, with respect to any preset outside diameter, the control is carried out by using the ratio of amount of fluctuation in the preset outside diameter, whereby more precise extension can be effected. Though it has conventionally been difficult to carry out precise extension by use of an electric furnace elongating machine alone, it can be effected by the above-mentioned method, whereby precision elongating can be performed by the electric furnace elongating machine alone, thus making it possible to precision-expand a glass preform having a greater outside diameter in a single step. Based on the above-mentioned value, proportional control, differential control, and integral control can be effected in combination.
Preferably, the moving speed-of the first holding section and/or second holding section is controlled according to the following expression (I), (II):
Vd/Ud=(R2/R1)Kxe2x80x83xe2x80x83(I)
Vu/Uu=(R2/R1)xe2x88x92Kxe2x80x83xe2x80x83(II)
Here, Vd is a target moving speed [mm/min] of the first holding section under control, Ud is a preset moving speed [mm/min] of the first holding section, Vu is a target moving speed [mm/min] of the second holding section under control, Uu is a preset moving speed [mm/min] of the second holding section, and K is a control coefficient (any positive constant).
When the respective moving speeds of the individual holding sections are controlled by these expressions, precise extension can favorably be effected by use of the electric furnace elongating machine even when the glass preform outside diameter before extension and the target elongated outside diameter vary.
Also, the method of elongating a glass preform in accordance with the present invention may be characterized in that the target moving speed of the first holding section and/or second holding section is controlled according to a value [(R2/R1)xe2x88x921] obtained from the reference value R1 and actually measured value R2.
The precision elongating by use of an electric furnace elongating machine can favorably be carried out regardless of the outside diameter of the glass preform in this case as well. In particular, it is advantageous in that feedback can be effected rapidly. Based on the above-mentioned value, proportional control, differential control, and integral control can be effected in combination.
Here, it is preferred that the target moving speed of the first holding section and/or second holding section be controlled according to the following expression (III), (IV). The respective meanings of individual symbols are similar to those in the case of the above-mentioned expression (I), (II).
Vd/Ud=1+K[(R2/R1)xe2x88x921]xe2x80x83xe2x80x83(III)
Vu/Uu=1xe2x88x92K[(R2/R1)xe2x88x921]xe2x80x83xe2x80x83(IV)
Expressions (III) and (IV) are obtained by subjecting the above-mentioned expressions (I) and (II) to Taylor""s expansion, respectively, and then omitting the resulting quadratic and higher-order terms. When the respective target moving speeds of the individual holding sections are controlled by such expressions, then precision elongating by use of an electric furnace elongating machine can favorably be carried out regardless of the outside diameter of the glass preform. Further, since the control expressions are not represented by functions of powers of (R2/R1) but by simple linear functions, arithmetic operations concerning the control can be carried out rapidly, whereby feedback can be performed promptly.
Preferably, the above-mentioned control coefficient K in expressions (I) to (IV) is set to a value within the range of 50 to 500. As a result, the amount of fluctuation in outside diameter of the manufactured elongated body can be made smaller, whereby an optical, fiber having a more uniform outside diameter can be obtained. If the control coefficient K is less than 50, the amount of change in the target moving speed (hereinafter also referred to as target speed) Vd, Vu will be so small that effective results may not be obtained by the control, thus increasing, on the contrary, the amount of change in outside diameter of the elongated body manufactured. If the control coefficient exceeds 500, by contrast, though the amount of fluctuation in outside diameter in the tapered region of the glass preform in the process of elongating can be made more constant, the amount of fluctuation in outside diameter of the manufactured elongated body may increase, on the contrary, under the influence of the unevenness in outside diameter inherent in the glass preform. In the above-mentioned range, it is particularly preferred that the control coefficient K be within the range of 100 to 250. Within the latter range, the amount of, fluctuation in outside diameter of the manufactured elongated body can be made very small.
Also, it is preferred that the target speed Vd is restricted so as to fall within the range represented by the following expression (V):
(100xe2x88x92X1)Ud less than 100Vd less than (100+X1)Udxe2x80x83xe2x80x83(V)
where X1 is any constant within the range of 10 to 100.
When the target speed Vd is set so as to fall within a predetermined range with respect to the preset speed Ud, then the outside diameter of the elongated body manufactured can be prevented from failing to diverge. Specifically, if the target speed Vd calculated by the above-mentioned expression (I) or (III) is lower than the lower limit of the restriction range, then the lower limit value of the restriction range is employed as the target speed Vd; whereas, if it exceeds the upper limit of the above-mentioned restriction range, then the upper limit value of the restriction range is employed as the target speed Vd.
Here, if the target speed Vd is restricted such that X1 less than 10, then there occurs a state where the target speed Vd hardly changes with respect to the preset speed Ud, thereby failing to yield effective results by control and increasing, on the contrary, the amount of change in outside diameter of the elongated body manufactured. If the target speed Vd is restricted such that X1 greater than 100, by contrast, then the target speed Vd varies too much, so that the control system diverges (overshoots), whereby the outside diameter of the elongated body manufactured would not converge at a predetermined value.
Similarly, it is preferred that the target speed Vu is restricted so as to fall within the range represented by the following expression (VI):
(100xe2x88x92X2)Uu less than 100Vu less than (100+X2)Uuxe2x80x83xe2x80x83(VI)
where X2 is any constant within the range of 10 to 100.
Consequently, in totally the same manner as in the above-mentioned case of the first holding section, the target speed Vu is set within a predetermined range with respect to the preset speed Uu, whereby the outside diameter of the elongated body manufactured is prevented from failing to diverge. Here, X1 and X2 can be determined independently from each other.
The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.